1. Technical Field
The present invention relates generally to ignition coils for developing a spark firing voltage that is applied to one or more spark plugs of an internal combustion engine.
2. Discussion of the Background Art
Ignition coils are known for use in connection with an internal combustion engine such as an automobile engine, and which include a primary winding, a secondary winding, and a magnetic circuit. The magnetic circuit conventionally may comprise a cylindrical-shaped, central core extending along an axis, located radially inwardly of the primary and secondary windings and magnetically coupled thereto. One end of the secondary winding is conventionally configured to produce a relatively high voltage when a primary current through the primary winding is interrupted. The high voltage end is coupled to a spark plug, as known, that is arranged to generate a discharge spark responsive to the high voltage. The spark plug, however, operates as a noise generating source when such spark events occur, which may result in radio frequency interference (RFI). Separate mount ignition coils are generally connected to the spark plug through a spark plug cable, which has a minimizing effect on the RFI, due to its inherent inductance characteristics. Some types of ignition coils do not use ignition cables. For example, a relatively slender ignition coil configuration is known that is adapted for mounting directly above a spark plug--commonly referred to as a xe2x80x9cpencilxe2x80x9d coil. Such ignition coils, therefore, do not utilize ignition cables. Accordingly, the problem of RF noise presents particular challenges in the design of xe2x80x9cpencilxe2x80x9d (i.e., direct mount) coils.
One approach taken in the art is disclosed in U.S. Pat. No. 5,603,307 issued to Morita et al. Morita et al. disclose a buffer coil having an inductance that is much smaller than that of either the primary winding or the secondary winding connected in series with one of them. Morita et al., however, does not appear to disclose that the buffer coil is outside of the magnetic circuit described above, and is therefore not believed to be as effective in suppressing RF noise as may be possible.
Another approach taken in the art involves connecting a wire wound resistor in series with the secondary winding at the high voltage end of the secondary winding. Incorporating the resistor into the ignition coil adds complexity and introduces mechanical stress inasmuch as the substrate on which such wire wound resistors are available have a different coefficient of thermal expansion that conventional encapsulants (e.g., epoxy potting material) used in potting ignition coils. This mismatch causes the stresses during operation (thermal cycling) of the ignition coil, which in turn may lead to failure (i.e., failure of the encapsulant material, which leads to imperfect insulation).
Accordingly, there is a need for an improved ignition apparatus that minimizes or eliminates one or more of the problems as set forth above.
An object of the present invention is to solve one or more of the problems as set forth above. An ignition apparatus according to the present invention overcomes shortcomings of the conventional ignition apparatus by including a suppression device disposed outside of a magnetic circuit and comprising a suppression winding wound on the spool connected in series with the secondary winding. The foregoing approach eliminates thermal stresses associated with the conventional ignition coils that include a wire wound resistor. In addition, the present invention is less costly to implement compared to the use of the wire wound resistor. Being outside of the magnetic circuit allows the suppression device to present an inductance that is electrically distinct from the inductance associated with the secondary winding-this improves the suppression device""s ability to reduce RF noise.
An ignition apparatus according to the present invention comprises a magnetically permeable central core having a main axis, primary and secondary windings outwardly of the core. The secondary winding is wound on a secondary winding spool formed of magnetically non-permeable material. The core and the windings being included in a magnetic circuit. The apparatus further includes a suppression device disposed outside of the magnetic circuit comprising a suppression winding also wound on the spool and that is connected in series with the secondary winding.
A method of making an ignition apparatus is also presented.